Mongolian sumo grand champion Harumafuji speaks to journalists as investigations begin that he assaulted a rival opponent at a party with a bottle. Kyodo News via AP

One of the grand champions of sumo wrestling has pulled out of a competition after breaking another fighter's skull with a beer bottle.

Harumafuji, who belongs to the Isegahama stable, assaulted Takanoiwa, 27, also a professional sumo wrestler competing in the makuuchi top division, after getting into an argument while drinking in late October, according to informed sources.

The Japan Sumo Association, which imposes strict rules on wrestlers, said 33-year-old Harumafuji will sit out the 15-day sumo event, which started Sunday, while the investigation takes place. There has been no confirmation that the head injury and allegation are linked, but officials are investigating.

The grand champion apologised publicly but did not confirm the circumstances of the incident.

"As for Takanoiwa's injuries, I apologize deeply for causing trouble for stable master Takanohana, people affiliated with Takanohana stable, the Sumo Association and my stable master", the wrestler told local reporters Tuesday.

Wrestlers are expected to not show emotion after a victory and a rigid hierarchy exists.

But this case is far from the first time that the sumo world has been hit by scandal and reports of violence outside the ring.

In June 2007, a stable master and his three wrestlers were convicted over a bullying-death of a 17-year-old junior wrestler.

Back in 2010, the sport was rocked by alleged links between sumo wrestlers and yakuza crime syndicates. Both men are from Mongolia, a country that dominates the sport.